This invention relates to nuclear reactors and has particular relationship to reactors in which a coolant, typically water at about 600.degree. F. and 2000 pounds per square inch, is circulated through the nuclear core. In the interest of aiding the understanding of this invention by dealing with a concrete reactor, this application deals predominantly with pressurized water reactors. However, the invention is applicable to reactors of other types and to the extent that it applies to the other reactors such application is within the scope of equivalents of this invention. The core includes the fuel and is disposed in the lower region of the reactor vessel. In addition, the reactor includes control-rod assemblies. Control-rod assemblies include control rods, supports for the control rods, drive rods for the control rods and guides for the control rods and, in some reactors, other related components. The core with its fuel assemblies is usually referred to as the lower internals of the reactor. The guides and other fixed parts above the core are referred to as the upper internals of the reactor. The control rods are movable between the upper internals and recesses or thimbles in the core by drive rods.
The control rods are mounted in clusters on their supports. There are rod control clusters (RCC's) in which the rods have a high absorption cross-section for neutrons. These clusters are used to reduce the power or shut down the reactor and are moved between the core and their guides a number of times during the fuel cycle of a reactor. There are so-called gray control rods of substantially lower neutron absorption cross-section than the high-absorption RCC's and they serve for load follow or to moderate or control the power of the reactor. Gray rods are moved, during their life times between the core and the guides many times (typically 5,600), substantially more than the high-absorption RCC's, during the fuel cycle of a reactor. There are water-displacement rod clusters (WDRC'S). These rods are of about the same diameter as the RCC's. The WDRC's displace the water in the thimbles which do not receive RCC's or gray rods. Their function is to maintain the level of the soluble neutron poison low during earlier operation of the reactor and to permit the level of the soluble neutron poison to remain low during the later operation of the reactor. Such clusters are in the core during earlier operation, typically during the first 60% of the fuel cycle, and are raised into the upper internals during the remainder, typically 40% of the fuel cycle. Typically the control rods are tubular. In this application and in its claims, the expression "control-rods", unless modified by wording signifying a specific type of rod, means any or all of the above-described control rods. There are nuclear reactors which do not include gray control rods or WDRC's and there are reactors in which the control is effected by blades instead of rods. Such reactors are within the scope of equivalents of this invention.
Typically, the RCC's and the gray rods in their clusters are carried by cruciform supports and are movable inside of hollow cruciform guides. These guides conventionally have holes or slots through which coolant flows. The WDRC's are not so protected. A large number of these tubes are movable in rectangular or square guides which conventionally have holes through which the coolant flows. During the later part of the fuel cycle, the WDRC's are moved into and remain in perforated guides. All guides are part of the upper internals of the reactor.
The coolant flows vertically through the core and into the upper internals. The outlet nozzles of the vessel are disposed between the ends of the upper internals and they conduct the coolant horizontally. In nuclear reactors in accordance with the teachings of the prior art, the coolant then flows generally horizontally or transversely through the control-rod assemblies in passing to the nozzles. Typically the vertical flow of the coolant through the core has a velocity of about 16 ft/sec. The flow through the nozzles has a velocity of about 50 ft/sec and the flow through some regions of the upper internals may be as high as 30 or 40 ft/sec. At these velocities, the coolant causes the vertical members and particularly the WDRC's, during the later part of the fuel cycle, to vibrate. The mechanisms which may cause the vibrations include vortex shedding, turbulent buffeting, fluid-elastic interaction, and cavitation.
It is an object of this invention to prevent failure of the components of the control-rod assemblies and particularly of the WDRC's by reason of the transverse flow of the coolant.